Cortana: everything you need to know about Microsoft's Siri rival

All the info on the new Windows Phone assistant

We knew Cortana was coming, but we didn't expect her to keep her codename: in the Halo games Cortana is an artificial intelligence that goes wrong.

It's a sign of Microsoft's current confidence that such things don't seem to matter: when Joe Belfiore asked Cortana whether she liked her name and appearance at this week's Build 2014, she replied that she resembled a halo — and she did it in a voice that sounded awfully like Jen Taylor, who plays Cortana in the video game.

So what is Cortana, and what can she do for you?

What is Microsoft Cortana?

Cortana is a personal digital assistant, a kind of silicon secretary who can help make your life easier. Instead of searching for things you ask Cortana - so if you want to know what the weather forecast is, how many pounds are in a kilo, who's winning the football or when Jim's birthday is, you'll ask and Cortana will give you the answer.

As Texas sang, she has a Halo. Will everyone adore her?

Microsoft says that Cortana will be socially intelligent, so for example you'll be able to set a reminder to ask somebody about something, and the reminder will pop up if you text, call or otherwise interact with that person.

How does it work?

Cortana can be voice activated or you can type your questions or commands, and she'll respond in kind: silently if you type and verbally if you talk.

She replaces the search function in Windows Phone and appears as a Live Tile on your device. Her intelligence comes via Bing, which has access to Tellme's natural language processing (which Microsoft acquired in 2007), the Satori knowledge repository and Microsoft's enormous cloud processing power.

Cortana is designed to learn about you and store what matters in her Notebook. That means she'll silence your phone during your favoured quiet times, warn you about travel issues and remind you that your friends owe you money.

Cortana promises to be a silicon secretary, an oracle and a travel companion

What's the Microsoft Cortana release date?

Cortana will be released "in the next few months" as a beta for Windows 8.1, and she will be US-only to begin with. The full version is planned for a late-2014 launch in the US, UK and China, with other countries getting Cortana in 2015.

For now, Cortana development appears to be limited to Windows Phone 8.1 - although an appearance on Windows and the Xbox One is surely a case of when, not if.

Is it better than Siri and Google Now?

As you might expect, Microsoft promises that Cortana will be the best thing ever: she'll take the best of Google Now's analysis and Siri's user-friendliness, bring them together and then take everything one step further.

Cortana won't just listen to you, she'll understand you, and she'll be aware of everything from your location to your personal preferences.

So when you ask her to book a hotel, she'll find the kind of hotel you like in the kind of area you like to stay in at the kind of price you want to pay. If you're in an airport she'll anticipate that you need a boarding card and will have it ready when you unlock your phone.

These are the kinds of things we're used to hearing from Apple and Google, yet they've not come to fruition... could Microsoft have stolen a march here?

Cortana will be available to third party developers: unlike Siri, which is limited to Apple's own apps and a few Apple-anointed services, Cortana could potentially work with any of your apps. Microsoft demonstrated Cortana working in Skype, Hulu Plus and Facebook.

That third-party support could be significant. Cortana could be a wine expert and a travel agent and a personal assistant and anything else you might need to make your everyday life a little bit easier. The vision is of Cortana as a smart, well-informed helper or friend. It'll be a few months before we discover if Microsoft can deliver on those promises.

Isn't Cortana just Microsoft Bob all over again?

Bob was years ahead of his time: he was made in 1996, when the world didn't have decent mobile broadband, let alone cloud computing and the enormous amounts of data Cortana can process. We can't wait to discover what Cortana has to say about him.